Lester awakens just as Spock's tricorder picks up faint life-form readings. Lester becomes unquiet and Dr. McCoy suggests that Kirk stay with her while the others leave to investigate the readings.

Kirk and Lester reminisce about their time together at Starfleet Academy, Lester still resenting her inability to rise to a captaincy. When Kirk examines an apparatus in the room, Lester activates it. It traps Kirk into position on one side of it. Lester takes a place alongside Kirk on the apparatus and effects a life-energy transfer, each from one body into the other.

Lester (in Kirk's body) discloses to Kirk (in Lester's body) her plan to command the Enterprise, as well as her willingness to kill. She starts to strangle Kirk but is interrupted as the others return. McCoy reports that the rest of the staff on the planet are dead of exposure to celebium, though Coleman says that this is unclear, a disagreement that will affect the choice of treatment. The survivors beam back to the Enterprise. Lester, in Kirk's body, orders that a medical team stand by in the transporter room for Kirk in Lester's body.

In sickbay, Lester and Dr. Coleman discuss Kirk, whom they want to keep from reawakening. Coleman knows Lester's plans, and in fact knows both that celebium was the lethal agent and that Lester had caused the deaths by sending the personnel to where the celebium shielding was weak. On the surface of Camus II, Coleman had kept the rest of the landing party apart to give Lester time enough to kill Kirk, but he refuses to induce Kirk's death.

McCoy arrives and is surprised to see the captain in sickbay. Lester, in Kirk's body, transfers responsibility for the patient's care to Dr. Coleman, despite McCoy's strenuous protests. The patient regains consciousness, but Coleman orders Chapel to administer a sedative.

The impostor Kirk orders Sulu to execute a course change for a hospital on Benecia Colony, even though Spock points out that a course for Starbase 2 would provide a better radiation treatment without delaying the rendezvous with the Potemkin at Beta Aurigae. The new Kirk reacts sternly to being informed of the consequences of his orders.

The captain next meets with McCoy in Kirk's quarters, who defends his qualifications to treat Lester and has discovered that Coleman was relieved of duty as a chief medical officer on a starship due to severe incompetence. The captain says his decisions stand but McCoy uses his authority to order the captain to a medical examination based on "emotional instability and erratic mental attitudes since returning from that planet." The captain calls this revenge, but the confrontation is interrupted as he is recalled to the bridge.

In sickbay, Kirk as Lester again regains consciousness and calls for McCoy. Dr. Coleman says he is in charge and tells Nurse Chapel that Kirk's claims are symptoms of a paranoia that has been developing for six months. He tells Kirk, "You are insane, Dr. Lester." He orders the nurse to keep Dr. Lester under constant sedation.

Kirk, in Lester's body and working not to seem paranoid to Nurse Chapel, asks to meet with McCoy or Spock, but is alarmed to hear in passing of the course change ordered for the Enterprise. After Chapel leaves, Kirk escapes from his restraints by breaking a glass and using it to cut them.

Nearby, McCoy discusses with Spock his plans to run tests on "Kirk". News of the captain's aberrant behavior is spreading around the ship, and both of them are convinced that a rapid-onset mental illness began during Kirk's brief time alone with Lester on Camus II. The patient approaches the two – but the captain had arrived a moment earlier. Lester orders Kirk to be placed in isolation with a twenty-four-hour watch.

Soon, however, Spock arrives at the cell to question the prisoner. LieutenantGalloway concedes Spock's point that isolation orders have never applied to the Enterprise's senior staff. Kirk calmly explains to Spock the technology of life-entity transfer, "accomplished and forgotten long ago on Camus II." When Spock protests that Starfleet requires objective evidence, Kirk describes events from their common past, such as their encounter with the Tholians and also the Vians, then finally, he invites the Vulcan mind meld. Performing, then breaking, the meld, Spock is convinced and asks Kirk to come with him. Galloway tries to block this. Spock disables him with a Vulcan neck pinch, but not before Galloway can call for help.

The captain passes McCoy's physical, but McCoy insists on performing the Robbiani dermal-optic test to compare to a previous test. This, too, reveals nothing. Then they hear on the intercom that the prisoner has escaped. The captain goes to the cell, Spock surrenders, and the captain broadcasts throughout the ship a call for an immediate court martial of Spock on the charge of mutiny.

The court martial convenes in a briefing room. Scott interrogates Spock, who describes his telepathic evidence that Kirk and Lester have exchanged bodies. Scott tells Spock that Starfleet Command will need more concrete proof than that. Later, McCoy testifies that the captain's physical and mental state are as they were when he assumed command of the Enterprise. Spock's logic compels Lester to call Kirk out of isolation to testify. Kirk describes the life-entity transfer, but Lester interrogates him in such a way so as to ridicule him.

Lester accuses Spock of inventing the life-entity transfer controversy to assume the captaincy. Spock asserts his intent to reveal the truth and oppose the captain. Lester regards this as an overt confession of mutiny, but as she makes this clear, she sounds increasingly irrational. She declares a recess, followed immediately by the vote.

In the corridor, McCoy and Scott agree that the captain's state of mind is unprecedented, Scott admitting to never having seen the captain "red-faced with hysteria." Scott asks McCoy what the captain will do on losing the vote, concluding that "we'll have to take over the ship" and conceding that that too will be mutiny. But Lester is recording the conversation and extends the mutiny charge, and orders the death penalty to them. Chekov and Sulu protest that the death penalty is expressly forbidden, except for General Order 4, which has not been violated by any Enterprise crewmember. The enraged Lester angrily orders that the other senior officers return to their posts and has Spock, Scott, McCoy, and Kirk detained in a holding cell by Lemli to await execution.

Lester schedules a group execution on the hangar deck, with interment to take place on Benecia, but Sulu and Chekov take their hands away from their helm and navigation consoles in defiance. But just then, the life-entity transfer temporarily reverses, as Kirk senses it briefly in the holding cell. Lester runs to meet Dr. Coleman in the archaeology lab to tell him the transference is weakening. Coleman says the only solution is to kill Lester's body but again refuses to do so personally. However, Lester says Coleman is complicit in many murders and now has no choice. He gives Lester a phaser and prepares a doubly lethal hypospray and the two proceed to the detention cell.

"Her life could have been as rich as any woman's. If only... if only..."

Lester disables the cell's force field and orders Kirk to come out, to be the first to be executed, lying that the prisoners would be moved to different cells, supposedly to prevent further conspiracy. But before she can kill Kirk with the hypospray, the life-entity transfer breaks and reverses for good, and Lester weeps over her defeat. Kirk lets Coleman accompany Lester back to sickbay to care for her and laments her sadness.

"Captain's log, stardate 5928.5. The Enterprise has received a distress call from a group of scientists on Camus II who were exploring the ruins of a dead civilization. The situation is desperate. Two of the survivors of the expedition are the surgeon, Dr. Coleman, and the leader of the expedition, Dr. Janice Lester."

(Log entry made by Janice Lester, acting as Captain James T. Kirk) "James Kirk is returning to consciousness in the body of Janice Lester. The Enterprise is proceeding to its next mission, on the course set for it before I took over command. Now the years I spent studying every single detail of the ship's operation will be tested. With a little experience, I will be invulnerable to suspicion. At last I attained what is my just due – command of a starship. All the months of preparation now come to fruition."

(Log entry made by Captain James T. Kirk, acting as Janice Lester) "Captain's log, stardate unknown. I have lost track of time. I am still held captive in a strange body and separated from all my crew."

(Log entry made by Janice Lester, acting as Captain James T. Kirk) "Captain's log, Stardate 5930.3. The results of Dr. McCoy's examination have given me complete confidence in myself. My fears are past. I shall function freely as the captain. I am the captain of the Enterprise, in fact."

"I loved you. We could've roamed among the stars."
"We'd have killed each other."
"It might have been better."

- Lester and Kirk, before their life-energy transfer

"Believe me, it's better to be dead than to live alone in the body of a woman."

- Lester, inside Kirk's body

"Youth doesn't excuse everything, Doctor McCoy."

- Lester as Kirk, in the transporter room

"Love? Him?!? I loved the life he led. The power of a starship commander. It's my life now!"

- Lester as Kirk, to Coleman

"You are closer to the captain than anyone in the universe."

- Kirk as Lester, to Spock

"You are not Captain Kirk. You have ruthlessly appropriated his body, but the life entity within you is not that of Captain Kirk. You do not belong in command of the Enterprise, and I will do everything in my power against you."

- Spock at his hearing, to Lester as Kirk

"You have heard the statements you put into the record. Do you understand the nature of it?"
"I do, sir. And I stand by it."
"It is mutiny!"

- Lester as Kirk and Spock

"On the basis of these statements, I call for an immediate vote, by the powers granted to me as Captain of the Enterprise. A recess is declared, to be followed by a vote."
"Yes, sir. An immediate vote before our chief witness can be left to die on some obscure planet with the truth locked away inside of her."
"Silence! You will be silent!! A recess has been declared. There will be no cross-discussion. When I return, we will vote on the charge of mutiny. The evidence presented here is the only basis of your decision."

- Lester, as Kirk, and Spock

"Doctor, I've seen the captain feverish, sick, drunk, delirious, terrified, overjoyed, boiling mad. But up to now I have never seen him red-faced with hysteria."

- Scott to McCoy, outside the hearing room

"Headquarters has its problems, and we have ours. And right now, the captain of the Enterprise is our problem."

- Scott, to McCoy

"We're talking about a mutiny, Scotty."
"Aye. Are you ready for the vote?"
"Yes, I'm... I'm ready for the vote."

- McCoy and Scott, before returning to the hearing room

"The death penalty is forbidden. There is only one exception."
"General Order 4! It has not been violated by any officer on the Enterprise!"

- Sulu and Chekov, on the death penalty

"The bridge is where you belong."

- Scott to Kirk as Lester, in the brig

"Oh, I'm never going to be the captain, never... kill him..."
"You are... you are as I loved you."

- Janice Lester and Dr. Coleman

"I didn't want to destroy her."
"I'm sure we all understand that, captain."
"Her life could have been as rich as any woman's. If only... if only..."

Because Star Trek: The Original Series was canceled instead of coming to a natural end, and the idea of a series finale was not as popular in the 1960s, "Turnabout Intruder", despite being the series finale of TOS, is closer to a normal episode and does not have the "finale" touches.

This episode was inspired by Thorne Smith's 1931 comedic novel Turnabout, in which a husband and wife switch bodies. A film version, directed by Hal Roach, appeared in 1940. The novel also inspired a short-lived 1979 TV series that starred Sharon Gless and John Schuck.(citation needed • edit)

In James Blish's novelization of "Turnabout Intruder" in Star Trek 5, Dr. Arthur Coleman's first name is "Howard" – probably from an early draft of the script. At the end of the episode, Kirk muses about Janice Lester, "Her life could have been as rich as any woman's, if only... if only...." In Blish's rendition, Spock finishes the sentence, adding: "If only she had been able to take pride in being a woman."

After two years on the series, Roger Holloway finally got to speak dialogue – all of two words: "Aye, sir." His character's name (Lemli) was the same as William Shatner's license plate at the time, a mixture of his daughters' names (i.e., Leslie, Melanie, and Lisabeth).

The very last Enterprise crew member to be seen in the original series is Scotty. As he, Kirk, and Spock enter the turbo-elevator at the end of Act IV, a glimpse of his forearm, grasping the control handle, is visible before the doors close.

There is a detailed account of the filming of this episode in the 1975 book Star Trek Lives!. Co-author Joan Winston had the opportunity to spend six days on the set while "Turnabout Intruder" was being shot. Winston wrote that Shatner was very ill with the flu at the time, and had considerable difficulty in picking up and carrying Sandra Smith, the actress who played Dr. Lester, for take after take. At one point, he said, "You know I love you, baby, but you've got to lose about six inches off that ass," which brought down the house.

Joan Winston also recalled many amusing anecdotes that took place during the shooting. For example, William Shatner flubbed the line, "Spock, give it up. Come back to the Enterprise family. All charges will be dropped. And the madness that overcame all of us on Camus II will fade and be forgotten." Instead, he blurted out, "Spock, it's always been you, you know it's always been you. Say you love me too."

Shatner clashed with Director Herb Wallerstein, when Wallerstein wanted Kirk to exit a scene via what was established as a wall. [1]

Even as filming was wrapping up, crew members were dismantling the Enterprise sets. Filming was completed on 9 January1969. Production went one day over schedule, resulting in seven filming days. (Inside Star Trek: The Real Story)

The final scene ever filmed for the original series was of William Shatner and Sandra Smith in front of the alien transference machine.(citation needed • edit)

This installment opens with the same music cue as the previous three episodes, taken from the opening titles of "Spock's Brain". In common with the immediately prior entry to this one, this episode ends with the closing music cue from "Elaan of Troyius".

A scheduled airdate of 28 March1969 was preempted by news coverage of the death of former president Dwight D. Eisenhower. This episode was not aired until 3 June1969 for that reason. Due to being rescheduled, the episode missed the deadline for Emmy nominations. (Smithsonian magazine, May 2016 issue, p. 59) It was the last installment of the franchise to air in the 1960s.

Although the BBC's first run airdate order during 1969-1971 was very different to NBC's, this was also the final episode screened in the UK, on 15 December1971.

Bjo and John Trimble were very impressed by this episode. "[It] was very good; it might have won an Emmy for William Shatner," they appreciatively speculated. (Smithsonian magazine, May 2016 issue, p. 59)

Critic Scott Mantz considered the episode's final moments to be significant. "There is something somewhat apropos," he said, "about the last words of the last episode, 'Turnabout Intruder': 'Her life could have been as rich as any woman's. If only... If only.' And then Kirk walks off." (Smithsonian magazine, May 2016 issue, p. 59)

Although this was the last episode of the Original Series to be filmed and aired, this episode has a lower stardate than the previous episode, "All Our Yesterdays".

Janice Lester, in the body and voice of Captain Kirk, makes a captain's log entry, discussing the status of the conspiracy.

As a subtle hint that all is not right with Kirk, whenever Lester/Kirk refers to himself, he uses the more formal "Captain Kirk" instead of the usual "Kirk". (e.g. "Captain Kirk to the Enterprise" instead of the usual "Kirk to Enterprise," "Captain Kirk out" instead of "Kirk out," etc.)

This episode is subtly referenced in TNG: "Legacy". In that episode, Jean-Luc Picard mentions in his captain's log that "The Enterprise has bypassed its scheduled archaeological survey of Camus II...", referencing the same planet that this episode begins on. This was mentioned because, with the airing of its eightieth episode, "Legacy", Star Trek: The Next Generation officially "bypassed" the series-run of Star Trek: The Original Series.(citation needed • edit)